netdev - Re: [PATCH 1/1] net: Add SO_REUSEPORT_LISTEN

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Message-ID: <1458925242.6473.41.camel@edumazet-glaptop3.roam.corp.google.com>
Date:	Fri, 25 Mar 2016 10:00:42 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Craig Gallek <kraigatgoog@...il.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [PATCH 1/1] net: Add SO_REUSEPORT_LISTEN_OFF socket option as
 drain mode

On Fri, 2016-03-25 at 12:31 -0400, Craig Gallek wrote:

> I believe the issue here is that closing the listen sockets will drop
> any connections that are in the listen queue but have not been
> accepted yet.  In the case of reuseport, you could in theory drain
> those queues into the non-closed sockets, but that probably has some
> interesting consequences...

It is more complicated than this.

Ideally, no TCP connection should be dropped during a server change.

The idea is to let old program running as long as :
1) It has established TCP sessions
2) Some SYN_RECV pseudo requests are still around

Once 3WHS completes for these SYN_RECV, children are queued into
listener accept queues.

But the idea is to direct all new SYN packets to the 'new' process and
its listeners. (New SYN_RECV should be created on behalf on the new
listeners only)

In some environments, the listeners are simply transfered via FD
passing, from the 'old process' to the new one.